Various channels (molecular sensors) are present in the terminal of nociceptive nerves, and various voltage-gated Na+ channels (Nav channels) and K+ channels (Kay channels) are present in nerve trunks. In addition, membrane potential (i.e., generator potential) is present in the nerve terminal by various channels. When such Nav channels are depolarized by generator potential in the nerve terminal, they play an important role in generating action voltage. Thus, the Nav channels play an important role in various diseases, including epilepsy (see Yogeeswari et al., Curr. Drug Targets, 5(7):589-602 (2004)), arrhythmia (see Noble D., Proc. Natl. Acad. Sci. USA, 99(9):5755-6 (2002)), myotonia (see Cannon, S. C. et al., Kidney Int., 57(3):772-9(2000)), ataxia (see Meisler, M. H. et al., Novartis Found Symp., 241:72-81 (2002)), multiple sclerosis (see Black, J. A. et al., Proc. Natl. Acad. Sci. USA, 97(21):11598-11602 (2000), and Renganathan, M. M. et al., Brain Res., 959 (2): 235-242 (2003)), irritable bowel syndrome (see Laird, J. M. et al., J. Neurosci., 22(19):8352-3856 (2002)), urinary incontinence and visceral pain (see Yoshimura, N. S., et al., J. Neurosci., 21(21): 8690-8696 (2001)), depression (see Hurley, S. C. et al., Ann, Pharmacother, 36(5):860-873 (2002)), and pain (see Wood, J. N. et al., J. Neurobiol., 61(1):55-71 (2004)). Currently, ten Nav channels (Na1.1-1.9 and Nax) are found in humans. Among them, four channels (Na1.3, Na1.7, Na1.8 and Na1.9) are known to have a close connection with the transmission of pain signals, and thus are recognized as important analgesic targets.
There are a total of ten known Nav channels as summarized in Table 1 below. Among the ten channels, nine channels (Nav1.1-NaV1.9) form channels (see Goldin, A. L. et al., Annu. Rev. Physiol., 63:871-894 (2001)). Among them, Nav1.3, Nav1.6, Nav1.7, Nav1.8 and Nav1.9 are expressed in DRG.
TABLE 1PrimaryTTX IC-TypeGenetissue50 nMIndicationsNav1.1 SCN1ACNS/PNS10Pain, epilepsy, neurodegenerationNav1.2 SCN2ACNS10Neurodegeneration, epilepsyNav1.3 SCN3ACNS15Pain, epilepsyNav1.4 SCN4ASk.muscle 25MyotoniaNav1.5 SCN5AHeart2000ArrhythmiaNav1.6 SCN6ACNS/PNS6Pain, movement disordersNav1.7 SCN7APNS25Pain, disorder of neuroendocrine systemNav1.8 SCN8APNS50000PainNav1.9 SCN9APNS1000Pain
Particularly, Nav1.7 is known to be highly expressed mainly in dorsal root ganglia (DRG) and sympathetic ganglia (see Toledo-Aral, J. J. et al., Proc. Natl. Acad. Sci. USA., 94:1527-1532 (1997), and Rush, A. M. et al. Proc. Natl. Acad. Sci. USA., 103:8245-8250 (2006)). In DRG that are sensory ganglia, the Nav1.7 channel is expressed in A- or C-fiber neurons, but frequently distributed in small neurons having a deep connection with pain. Particularly, 85% of DRG are present in cells defined as nociceptors (see Djouhri, L. et al., J. Physiol., 546: 565-576 (2003)). This fact indicates that Nav1.7 has a close connection with pain.
The fact that the Nav1.7 channel has a close connection with pain is well demonstrated in the results of not only animal studies, but also human disease studies. The results of animal studies indicated that, when inflammation occurs, the gene transcript of Nav1.7 significantly increases and the expression of proteins also increases. This increase in transcript is believed to be attributable to an increase in NGF. The increased expression of Nav1.7 is believed to be the direct cause of an increase in excitability of sensory cells. In particular, when the gene of the Nav1.7 channel is removed or reduced, inflammatory pain is greatly reduced. However, animal studies do not indicate that the removal or reduction of the Nav1.7 channel gene reduces neuropathic pain. However, there are many evidences that Nav1.7 is involved in neuropathic pain in humans.
Examination results for lineages that feel severe pain or no pain give many answers to pain studies. Particularly, these results directly indicate the importance of Nav1.7 in causing pain. There are two types of inherited diseases that cause severe pain. In the case of erythromelalgia or erythermalgia among these diseases, severe pain is sometimes felt for a few hours when the body is slightly warm or takes exercises. In some cases, the skin becomes red, and the hand, the foot or the face swell. The results of genetic research indicated that SCN9A (the human gene name of Nav1.7) is present at chromosomal sites associated with diseases. Nine mutations of Nav1.7 were found until now. These mutations lower activation threshold or result in slow deactivation of the channel. Thus, these mutations can easily generate action potential even upon depolarization of some neurons (see Dib-Hajj, S D. et al., Trends in Neurosci., 30, 555-563:(2007)).
In the case of paroxysmal extreme pain disorder (PEPD) that is another inherited disease, pain is felt through life and caused when the bowels are evacuated or the anal region is stimulated. In addition to pain, the leg becomes red. As is known in the art, in PEPD, eight mutations occur in Nav1.7. These mutations occur mainly in sites that cause inactivation. The Nav channel has an inactivation ball in the linker between domains III and IV, and a peptide receiving region in the linker between the S5 and S6 segments of domains III and IV. Interestingly, mutations that cause PEPD all occur in these two regions. It appears that these cause a problem in the inactivation of Nav1.7. As expected, these mutations cause a problem in the inactivation of Nav1.7, resulting in slow deactivation of the channel (see Fertleman, C. R. et al., Neuron, 52, 767-774 (2006)). Thus, the amount of electric current that enters through the channel increases.
Still another inherited disease is congenital indifference to pain (CIP). This disease results from mutation of the Nav1.7 channel and exist in Pakistani and Chinese lineages. Persons suffering from this disease feel no pain (see Cox, J. J. et al., Nature, 444, 894-898 (2006)). Particularly, persons suffering from this disease do not feel almost all pains, including a pain caused by a burn, and organ pains (see Cox, J. J. et al., Nature, 444, 894-898 (2006)). CIP causes the loss of function of the Nav1.7 channel. Particularly, a mutation in this channel inhibits the expression of this channel. Thus, this channel is not expressed (see Cox, J. J. et al., Nature, 444, 894-898 (2006)). Interestingly, the knock-out of Nav1.7 does not influence other sensations (see Dib-Hajj, S D. et al., Trends in Neurosci., 30, 555-563 (2007)). However, it influences the olfactory sensation. This fact directly indicates that Nav1.7 does not overlap with other channels in pain transmission and the function thereof is not compensated for by other Nav channels.
As described above for the above diseases, when a mutation in the Nav1.7 channel causes a gain of function, severe pain is felt, and when it causes a loss of function, pain is relieved. This is a good clinical example that directly shows that the Nav1.7 channel is the major cause of pain. Thus, it is considered that an antagonist that inhibits this channel will naturally result in a pain-relieving effect.
However, if the Nav1.7 channel antagonist inhibits a plurality of Nav channels including the Nav1.7 channel, it can show adverse effects of various CNS disturbances, such as blurring of vision, dizziness, vomiting and depression. Particularly, if it inhibits the Nav1.5 channel, it can cause cardiac arrhythmia and heart failure, which threaten life. For these reasons, selective inhibition of the Nav1.7 channels is very important.
Pains can be largely classified into three: acute pain, inflammatory pain, and neuropathic pain. Acute pain plays an important protective function of maintaining the safety of organisms from stimuli that can cause tissue injury. Thus, it is generally temporary and intense. On the other hand, inflammatory pain can be longer lasting, and the intensity thereof further increases. Inflammatory pain is mediated by various substances that are released during inflammation, including substance P, histamine, acids, prostaglandin, bradykinin, CGRP, cytokines, ATP and other substances (see Julius, D. et al., Nature, 413 (6852):203-210 (2001)). The third pain is neuropathic and involves nerve injury or a nerve injury caused by viral infection. It causes reconstitution of circuits with neuron proteins to cause pathological “sensitization”, which can result in chronic pain that is lasting for several years. This type of pain does not provide an advantage of adaptability and is difficult to treat by current therapy.
Particularly, neuropathic pain and intractable pain are great medical problems that have not been solved. Several hundred million patients are suffering from severe pain that is not well inhibited by current therapeutic methods. Drugs that are currently used for the treatment of pain include NSAIDS, COX-2 inhibitors, opioids, tricyclic antidepressants and anticonvulsants. Neuropathic pain is particularly difficult to treat, because it does not well respond to opioids until a high dose is reached. Currently, gabapentin is most widely used as a therapeutic agent against neuropathic pain, but it is effective for 60% of the patients and is not greatly effective. This drug is generally safe, but is problematic in terms of sedative action at high doses.
Accordingly, studies on the discovery of new regulators of the Nav1.7 channel (see Wiffen, P. S. et al., Cochrane Database Syst. Rev 3., (2000); Guay, D. R., Pharmacotherapy, 21(9):1070-1081 (2001)) and the use thereof for the treatment of acute pain (see Wiffen, P. S. et al., Cochrane Database Syst. Rev3., (2000)), chronic acute (see Guay, D. R., Pharmacotherapy, 21(9):1070-1081 (2001)), inflammatory pain (see Gold, M. S., Proc. Natl. Acad. Sci. USA., 96(14): 7645-7649 (1999)) and neuropathic pain (Sandner-Kiesling, A. G. et al., Acta. Anaesthesiol Scand., 46(10):1261-1264 (2002)) have been actively conducted by global pharmaceutical companies, including Merck, AstraZeneca and the like (see WO-A-2005/013914; WO-A-2005/054176; WO-A-2008/118758; EP-A-1088819; WO-A-2009/012242; US2010/0197655 A1; U.S. Pat. Nos. 7,858,786 B2; 7,989,481 B2).
Accordingly, the present inventors have conducted studies on novel compounds, and as a result, have found that compounds having chemical structures different from those of sodium channel blockers reported to date have excellent sodium channel blocking effects, thereby completing the present invention. Compounds falling within the scope of the present invention mainly have sodium channel blocking activity, but it is not excluded that products produced by a special in vivo environment or a metabolic process after absorption of the compounds in vivo will act as agonists and exhibit effective pharmacological action.